Doped layers are often deposited during fabrication of various types of semiconductor devices. For example, during the fabrication of light emitting diodes (LED), p-doped layers such as p-GaN, and n-doped layers, such as n-GaN, can be deposited to form multi layered epitaxial structures configured to emit electromagnetic radiation. FIG. 1 illustrates a semiconductor structure 10 comprising an n-GaN layer 12 and a p-GaN layer 14 separated by a multi-quantum well MQW layer 16. The semiconductor structure 10 can be formed on a substrate 18 made of a suitable material, such as SiC, or sapphire.
The deposition process for forming the semiconductor structure 10 can include gaseous reactions, such as 3GaCl+3NH3→3GaN+2H2+3HCl or TMG+NH3→GaN+CHx+H2. Such deposition on a c-plane carrier substrate, typically results in the growth of a crystalline structure along the c-axis of the hexagonal crystal. An as deposited surface 20 (FIG. 2A) of the crystalline structure includes atomic bonds between Ga atoms 22 (FIG. 2A) and N atoms 24 (FIG. 2A). This crystalline structure is shown along the C-axis in FIG. 2A. The as deposited surface 20 (FIG. 2A) of the crystalline structure is usually very stable, as the atoms 22, 24 normally occupy the lowest free energy states during the deposition process.
As shown in FIG. 3, a p-pad contact 26 and an n-pad contact 28 can also be formed for applying a voltage differential across the n-GaN layer 12 and p-GaN layer 14 causing the multi-quantum well MQW layer 16 to emit electromagnetic radiation. In conventional LED fabrication processes, the n-pad contact 28 can be formed as a metal contact on the as-deposited surface 20 (FIG. 2A) of the n-GaN layer 12.
In some cases, semiconductor structures are formed using processes that result in crystal structures having an inverted configuration of atomic bonds between the Ga and N atoms along the c-axis of the crystal structure (in the direction towards the surface). The inverted configuration of atomic bonds can be formed by removing the substrate 18, thus exposing an inverted surface 30 (FIG. 2B). By way of example, this type of inverted surface 30 can be formed during fabrication of a VLED device, as described in U.S. Pat. No. 7,432,882, which is herein incorporated by reference.
The inverted surface 30 comprises a man-made surface, and the crystal structure of the surface 30 can be less stable than in an as deposited GaN surface 20 (FIG. 2A). As used herein stable refers to the uniformity of contact resistance when forming contacts. As a result of this, forming low resistance electrical contacts on an inverted GaN surface 30 is not well known in the art.
The present disclosure is directed to a method for fabricating a vertical light emitting diode (VLED) structure in which a carrier substrate is removed using a laser pulse to form an inverted surface. Low resistance contacts can then be formed on the inverted surface.